1. Field of the Invention
The present invention relates to luminescent screens and is also concerned with a method of fabrication of luminescent screens.
The screens under consideration in this invention are composed in particular of several layers of luminescent material in the form of grains which are deposited on a transparent support. As a general rule, this support consists of a glass substrate having parallel faces.
The luminescent material can be cathodoluminescent or, in other words, becomes luminescent when subjected to bombardment by an electron beam. Cathodoluminescent screens of this type are employed for example in cathode-ray tubes, x-ray image intensifiers and the like. By way of example, the luminescent material can also be electroluminescent or, in other words, becomes luminescent under the action of an electric field.
In the description which follows below, the problems to be solved and the solutions offered by the invention will be described in the case of cathodoluminescent screens employed in x-ray image intensifiers but it should clearly be understood that the invention applies to all the types of screens mentioned in the foregoing.
2. Description of the Prior Art (FIGS. 1 to 5)
An x-ray image intensifier is illustrated schematically in FIG. 1 of the accompanying drawings. This tube comprises a primary screen which has the function of converting the x-ray photons which it receives to light photons and then to photoelectrons. An electron-optical system (not shown in the figure) has the function of focusing the electron trajectories and producing an electron energy gain. Finally, a secondary cathodoluminescent screen effects the conversion of electrons to visible photons. It is this secondary screen which will be considered below.
FIGS. 2a and 2b of the accompanying drawings are transverse sectional views illustrating one form of construction of the secondary screen shown in FIG. 1.
On the glass substrate 1, there is formed a deposit 2 consisting of several layers of crystals of a cathodoluminescent substance, the first layer of which is in direct contact with the substrate. The last layer of crystals is covered with a metallic film 3 of aluminum, for example. This film serves to reflect to the observer the light produced within the screen and to apply an acceleration voltage to the incident electrons. FIG. 2b is an enlarged view of the screen zone which is surrounded by a circle in FIG. 2a.
By way of example, the cathodoluminescent substance employed can be silver-doped zinc sulfide. The diameter of the grains can vary for example between 1 and 3 microns according to the resolution which is sought.
The thickness of the glass substrate 1 is, for example, approximately 1 to 3 millimeters whereas the thickness of the luminescent material is approximately 10 microns.
It is known that the screens which have just been mentioned exhibit a halo phenomenon. When the screen is excited at a point which thus becomes luminous, light rings or halos are observed around this point on which they are centered. These halos are equidistant at a distance in the vicinity of double the thickness of the substrate and their intensity decreases as the observer moves away from the central luminous point.
This phenomenon is illustrated in FIGS. 3a and 3b of the accompanying drawings.
The screen shown in the profile view of FIG. 3a is subjected to an electron impact directed along the axis X--X'.
In FIG. 3b, there is shown the central luminous point resulting from this impact and three of the halos thus formed.
The explanation of this halo phenomenon will be recalled with reference to FIGS. 4 and 5 of the accompanying drawings.
In the sectional view of the luminescent screen shown in FIG. 4, the thickness of the metallic film 3 and of the layers of luminescent material 2 has been considerably increased in FIG. 4 with respect to the thickness of the substrate 1.
Any light ray generated in a grain A which is not in contact with the substrate passes through the substrate 1 as if it were a plate having parallel faces and produces an exit ray A.sub.1. The same applies to the light rays generated in grains which are in contact with the substrate. However, these light rays emerge from the grains at a location other than the point of contact of the grain with the substrate. This is the case with the ray B.sub.o which emerges from the grain B.
Consideration will now be given to the case of light rays which are emitted by the grain B in contact with the substrate and pass in addition into the substrate through the point of contact of the grain with said substrate.
In the case of these rays, the effect is the same as if they had been produced by a light source in intimate optical contact with the substrate. When the angle of incidence .theta. of these rays on the internal exit face of the substrate is smaller than the angle .theta..sub.o so that sin .theta..sub.o =1/n, where n is the refractive index of the substrate, these rays pass through the substrate towards the observer. This is the case of the rays B.sub.1 and B.sub.2 of FIG. 4. On the other hand, when the angle of incidence is larger than or equal to .theta..sub.o, a total reflection phenomenon takes place and the light rays such as the ray B.sub.3 in FIG. 4 are returned to the internal entrance face of the substrate. These rays are reflected laterally to a grain C which is in contact with the substrate and located at a distance: D=2e.multidot.tg.theta..sub.o .perspectiveto.2e from the grain B by reason of the fact that, with a glass substrate, n=1.5 and .theta..sub.o =42.degree.. As a result of diffraction or diffusion, the rays which impinge on the grain C are in some cases such as the ray C.sub.1 re-emitted towards the observer whilst other rays such as the ray C.sub.2 are reflected back to another grain D located at a distance approximately equal to 2e from the grain C.
This phenomenon continues from point to point until exhaustion of the light intensity and in all directions about the point B. A large number of rings thus appear around a light spot centered at B. These rings are relatively spaced at a distance D and have decreasing values of light intensity I, I.sub.1, I.sub.2, I.sub.3. The other points such as C or D exhibit halo phenomena which are less luminous than the halos oentered at the point B.
In FIG. 5, the substrate is shown in cross-section as well as the path of the light rays and in particular the rays which undergo total reflection. The variations in intensity I which are observed and correspond to the central spot and to the different halos are also shown in FIG. 5.
A number of known techniques which seek to suppress this halo phenomenon will hereinafter be described with reference to FIGS. 6 to 10. This phenomenon is highly objectionable since it gives rise to parasitic information which interferes with the useful information. Furthermore, the phenomenon produces a reduction in contrast of the screen.
The problem which arises is that the known techniques do not prove satisfactory. In particular, these techniques improve the contrast but produce a drop in luminous efficiency. Some of these techniques have the effect of reducing the resolution.
The present invention makes it possible to solve this problem and, as will hereinafter be explained in detail, makes it possible to obtain a screen which provides optimized contrast without excessive reduction of gain and without any impairment of resolution.